Present code division multiple access (CDMA) systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected receiver to determine the proper recipient of a data signal. These different data signals arrive at the receiver via multiple paths due to ground clutter and unpredictable signal reflection. Additive effects of these multiple data signals at the receiver may result in significant fading or variation in received signal strength. In general, this fading due to multiple data paths may be diminished by spreading the transmitted energy over a wide bandwidth. This wide bandwidth results in greatly reduced fading compared to narrow band transmission modes such as frequency division multiple access (FDMA) or time division multiple access (TDMA).
New standards are continually emerging for next generation wideband code division multiple access (WCDMA) communication systems as described in Provisional U.S. patent application Ser. No. 60/082,671, filed Apr. 22, 1998, and incorporated herein by reference. These WCDMA systems are coherent communications systems with pilot symbol assisted channel estimation schemes. These pilot symbols are transmitted as quadrature phase shift keyed (QPSK) known data in predetermined time frames to any receivers within range. The frames may propagate in a discontinuous transmission (DTX) mode. For voice traffic, transmission of user data occurs when the user speaks, but no data symbol transmission occurs when the user is silent. Similarly for packet data, the user data may be transmitted only when packets are ready to be sent. The frames are subdivided into fifteen equal time slots of 0.67 milliseconds each. Each time slot is further subdivided into equal symbol times. At a data rate of 30 KSPS, for example, each time slot includes twenty symbol times. Each frame includes pilot symbols as well as other control symbols such as transmit power control (TPC) symbols and rate information (RI) symbols. These control symbols include multiple bits otherwise known as chips to distinguish them from data bits. The chip transmission time (TC), therefore, is equal to the symbol time rate (T) divided by the number of chips in the symbol (N).
Previous studies have shown that multiple transmit antennas may improve reception by increasing transmit diversity for narrow band communication systems. In their paper New Detection Schemes for Transmit Diversity with no Channel Estimation, Tarokh et al. describe such a transmit diversity scheme for a TDMA system. The same concept is described in A Simple Transmitter Diversity Technique for Wireless Communications by Alamouti. Tarokh et al. and Alamouti, however, fail to teach such a transmit diversity scheme for a WCDMA communication system.
Another improvement in communication systems includes site selection diversity transmit power control (SSDT). The SSDT will be explained with reference to the flow diagram of FIG. 7. A mobile receiver frequently receives signals from multiple base stations. The mobile receiver calculates a signal-to-interference (SIR) ratio for each respective base station. These SIR values are applied to a selection circuit 720 in the mobile receiver. The selection circuit determines the base station with the greatest SIR and sends the identity of this base station to the control network 630 on a frame-by-frame basis. The control network then transmits the next data frame only on the selected base station. This reduces slow lognormal fading and interference within the communication system. SSDT further reduces interference at the mobile receiver and within the cell.
A problem arises with this simplified selection scheme when one or more of the base stations employ transmit diversity. This is because a good SIR may not correspond to a reduced bit error rate within the cell. An improved SIR due to STTD or other transmit diversity methods, therefore, may have a superior bit error rate compared to another base station having no diversity. Previous designs fail to offer or suggest a solution to SSDT base station selection when one or more base stations employ transmit diversity.